Topic 4 Genetic Engineering & Biotechnology

Question 1

4.1.1 State what eukaryotic cells are made of.

Answer 1

DNA and protein

Question 2

4.1.2 Define gene

Answer 2

A heritable factor that controls a specific characteristic, consisting of a length of DNA occupying a particular position on a chromosome (locus).

Question 3

4.1.2 Define allele

Answer 3

One specific form of a gene, differing from other alleles by one or a few bases only and occupying the same locus as other alleles of the gene.

Question 4

4.1.2 Define genome

Answer 4

The whole of the genetic information of an organism

Question 5

4.1.3 Define gene mutation

Answer 5

A change in the nucleotide sequence of a section of DNA coding for a particular feature

Question 6

4.1.4 Describe the consequences of a base substitution mutation

Answer 6

A different amino acid is placed in the polypeptide chain, which could either have little or no effect, or could have a major influence on an organism’s physical appearance.

Question 7

4.1.4 Describe the sickle cell anaemia mutation.

Describe the consequences of T replacing A.

Answer 7

In red blood cells, genes that create the haemoglobin are mutated. A mutation of base substitution occurs, where T replaces A.
This leads to haemoglobin having a sickle shape.

Question 8

4.2.2 Describe Meiosis

what type of division? what are the products?

Answer 8

Meiosis is a reduction division of a diploid nucleus, to form haploid nuclei. Process by which sex cells are made in the reproductive organs.

Question 9

4.2.1 What are gametes?

Answer 9

Sex cells

Question 10

4.2.1 Define Haploid Cells

Answer 10

Cells which contain a single set of chromosomes (e.g. gametes) 23 chromosomes

Question 11

4.2.2 Define homologous chromosomes

Answer 11

Chromosomes that share the same structural features (e.g. size, banding pattern, centromere position) and have the same genes and the same loci positions (although genes are the same alleles may differ)

Question 12

4.2.3 Outline the process of Meiosis

Answer 12

Meiosis leads to a diploid cell producing 4 haploid cells
Pre-meiosis, DNA replication
2 Cell divisions (Meiosis 1 and 2) 1st division - 1 cell -> 2 cells
2nd division 2 cells - 4 cells

Question 13

4.2.3 Explain the process of crossing over

Answer 13

Occurs in prophase 1, in which there is an exchange of genetic material between sister chromatids
1) 2 homologous chromosomes
2) homologous chromosomes twist
3) DNA is mixed from both parents
Allows for a mosaic of parent cell’s original chromosomes

Question 14

4.2.3 Describe the events of Meiosis I

Answer 14

Homologous chromosomes must first pair up in order to be sorted into separate haploid daughter cells
Prophase 1 - homologous chromosomes undergo a process called synapsis, whereby homologous chromosomes pair up to form a bivalent
- The homologous chromosomes are held together at points called chiasma
- Crossing over of genetic material between non-sister chromatids can occur at these points, resulting in new gene combinations
The remainder of Meiosis I involves separating the homologous chromosomes into separate daughter cells
- Metaphase 1 - homologous pairs line up along the equator of the cell
-Anaphase 1 - homologous chromosomes split apat and move to opposite poles
-Telophase 1 - cell splits into two haploid daughter cells as cytokinesis happens

Question 15

4.2.3 Describe the events in Meoisis II

Answer 15

The sister chromatids are divided into separate cells

• Prophase II - spindle fibres reform and reconnect to chromosomes
• Metaphase II - chromosomes line up along the equator of the cell
• Anaphase II - sister chromatids split apart and move to opposite poles
• Telophase II - cell splits in two as cytokinesis happens concurrently

Question 16

4.2.3 What type of cell division is Meiosis? What are the products of Meiosis?

Answer 16

Reduction Division

It results in the formation of four genetically distinct haploid daughter cells (Gametes)

Question 17

4.2.4 What is non-disjunction?

Answer 17

When homologous chromosomes fail to separate properly during anaphase (I or II), resulting in a gamete with too many or too few chromosomes

Question 18

4.2.4 Describe how non-disjunction can lead to Downs Syndrome

Answer 18

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21, leading to Down Syndrome

Question 19

4.2.5 What is a karyotype? How is one created?

Answer 19

A visual picture of an organism’s chromosomal genetic make-up, in which chromosomes are arranged into homologous paris and displayed according to their structural characteristics.

Question 20

4.2.5 Outline the process of karyotyping

Answer 20

• Harvesting cells (usually from foetus or white blood cells of adults)
• Chemically inducing cell division, then halting it during mitosis when chromosomes are condensed, thus viable (the stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids
• Staining and photographing chromosomes, before arranging them according to structure

Question 21

4.2.6 How is karyotyping performed? And for what reason?

Answer 21

Pre-natal karyotyping is often used to determine the gender of an unborn child, and to test for chromosomal abnormalities.

Question 22

4.2.6 Karyotyping by Chorionic Villus Sampling

Answer 22

• A tube is inserted through the cervix and a tiny sample of the chorionic vili (containing foetal cells) from the placenta is taken
• Can be done at ~11th week of pregnancy, with a slight risk of inducing miscarriage (~1%)

Question 23

4.3.1 Define Genotype

Answer 23

The allele combination of an organism

Question 24

4.3.1 Define Phenotype

Answer 24

The characteristics of an organism (determined by a combination of genotype and environmental factors)

Question 25

4.3.1 Define Dominant Allele

Answer 25

An allele that has the same effect on the phenotype, whether it is present in the homozygous or hetereozygous state

Question 26

4.3.1 Define Recessive Allele

Answer 26

An allele that only has an effect on the phenotype when present in the homozygous state

Question 27

4.3.1 Define Codominant Alleles

Answer 27

Pairs of alleles that both affect the phenotype when present in a heterozygote

Question 28

4.3.1 Define Locus

Answer 28

The particular position on homologous chromosomes of a gene

Question 29

4.3.1 Define Homozygous

Answer 29

Having two identical alleles of a gene

Question 30

4.3.1 Define heterozygous

Answer 30

Having two different alleles of a gene

Question 31

4.3.1 Define Carrier

Answer 31

An individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

Question 32

4.3.1 Define Test Cross

Answer 32

Testing a suspected heterozygote by crossing it with a known homozygous recessive

Question 33

4.3.3 Discuss how some genes have more than two alleles

Answer 33

Some genes have more than two alleles for a given trait (e.g. ABO blood group system)
The alleles which are not recessive may either:
- Share codominance (be expressed equally in the phenotype)
- Share incomplete dominance (neither is fully expressed in the phenotype, resulting in blending)
- Demonstrate a dominance order

Question 34

4.3.5 Explain how sex chromosomes control gender

Answer 34

Humans have 23 pairs of chromosomes, making a total of 46
The first 22 pairs are autosomes
The 23rd pair of chromosomes are heterosomes
Females are XX
Males are XY
The Y chromosome contains the genes for developing male sex characteristic, hence the father is always responsible for determining gender:
- If the male sperm contains the X chromosome the growing embryo will develope into a girl
- If the male sperm contains a Y chromosome the growing embryo will develop into a boy
- In all cases the female egg will contain an X chromosome

Because the X and Y chromosomes are a different size, they cannot undergo crossing over/recombination during meiois, thus ensureing that the gene responsible for gender always remains on the Y chromosome, ensuring a 50% of male or female embryo

Question 35

4.3.5 Define Autosomes

Answer 35

Each chromosome pair possesses the same genes and structural features

Question 36

4.3.5 Define Heterosomes

Answer 36

(sex chromosomes)

Determine gender

Question 37

4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome

Answer 37

The Y chromosome is much shorter than the X chromosome, and contains only a few genes, including the sex determination gene.
The X chromosome is much longer, and contains several genes not present on the Y chromosome, including genes for heamophilia and red-green colour blindness.

In human femals, only one of the X chromosomes remains active throughout life

• The other is packaged as heterochromatin to form a condesned Barr body
• This inactivation is random and individual to each cell, so heterezygous women will be a mosaic, expressing both alleles via different cells.

Question 38

4.3.7 Define sex linkage

Answer 38

When a gene controlling a characteristic is found on a sex chromosome (and so we associate the trait with a predominant gender)
- Sex-linked conditions are usually X-linked, as very few genes exist on the shorter Y chromosome

Question 39

4.3.8 Describe the inheritance of colour blindness and haemophilia as examples of sex linkage

Answer 39

Colour blindness and haemophilia are both examples of X-linked recessive conditions
The gene loci for these conditions are found on the non-homologous region of the X-chromosome (they are not present of the Y chromosome)
As males only have one allele for this gene they cannot be a carrier of the condition
This means they have a higher chance of being recessive and expressing the trait
Males will always inherit an X-linked recessive condition from their mother
Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents

Question 40

4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes

Answer 40

As human females have two X chromosomes they can either be homozygour or heterozygous
Males only have one X chromosome and are hemizygous

Question 41

4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles

Answer 41

An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier
Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves
Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers
Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater.

Question 42

4.2.12 Discuss Autosomal Dominance (pedigree chart)

Answer 42

All affected individuals must have at least one affected parent.
If two parents are unaffected, all offspring must be unaffected (homozygous recessive).
If two parents are affected, they may have offspring who are unaffected (if parents are heterozygous)

Question 43

4.2.12 Discuss Autosomal Recessive (pedigree chart)

Answer 43

If two parents show a trait, all children must also show the trait (homozygous recessive)
An affected individual may have two normal parents (if parents are both heterozygous carriers)

Question 44

4.2.12 Discuss X-linked Recessive (pedigree chart)

Answer 44

If a female shows the triat, so much all sons as well as her father
The disorder is more common in males.

Question 45

4.4.1 Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA

Answer 45

PCR is a way of producing large quantities of a specific target sequence of DNA.
It is useful when only a small amount of DNA is available for testing.
Occurs in a thermal cycler and involves a repeat procedure of 3 steps
1) Denaturation: DNA sample is heated to separate into two strands
2) Annealing: DNA primers attach to opposite ends of the target sequence
3) Elongation: A heat-tolerant DNA polymerase copies the strands

One cycle of PCR yields two identical copies of the DNA sequence

Question 46

4.4.2 Describe the process of gel electrophoresis

Answer 46

This is a technique used to separate fragments of DNA according to size

• Samples of fragmented DNA are placed in wells of agarose gel
• The gel is placed in a buffering solution and an electrical current is passed across the fel
• DNA, being negatively charged (due to phosphate), moves to the positive terminus (anode).
• Smaller fragments are less impeded by the gel matrix and move faster through the gel
• The fragments are thus separated according to size
• Size can be calculated by comparing against a known industry standard.

Question 47

4.4.3 What is DNA profiling? What process is used in it?

Answer 47

DNA profiling is a technique by which individuals are identified on the basis of their respective DNA profiles.
With the non-coding region of an individual’s genome, there exists satellite DNA- long stretches of DNA made up of repeating elements called short tandem repeats
These repeating sequences can be excised to form fragments, by cutting with a variety of restriction endonucleases (which cut DNA at specific sites)
As individuals all have a different number of repeats in a given sequence of satellite DNA, they will all generate unique fragment profiles.
These different profiles can be compared using gel electrophoresis.

Question 48

4.4.4 Describe the application of DNA profiling to determine paternity and also in forensic investigation

Answer 48

• A DNA sample is collected (blood, saliva, semen etc.) and amplified using PCR
• Satellite DNA (non-coding) is cut with specific restriction enzymes to generate fragments
• ## Individuals will have unique fragment lengths due to the variable length of their short tandem repeats (STR)